Shallow trench isolation structures are used in semiconductor device fabrication to isolate active areas of the semiconductor substrate. Typically, shallow trench isolation structures are formed by etching the top surface of the semiconductor substrate so as to form trenches that extend into the semiconductor substrate. These trenches surround areas of the semiconductor substrate that are commonly referred to as “active areas.”
Semiconductor fabrication processes typically require that adjoining devices within the semiconductor substrate be electrically isolated to a particular depth within the substrate, referred to as “isolation depth.” Isolation depth is the minimum depth at which adjoining active regions must be separated in order to prevent current leakage in excess of design specifications for a particular product. In conventional semiconductor fabrication processes trenches are etched to a depth that is equal to or that is slightly greater than the isolation depth. A layer of oxide is then deposited such that it fills the trenches. A chemical mechanical polishing process is performed, forming a dielectric plug that fills the trenches so as to isolate adjoining active areas.
The trenches of conventional shallow trench isolation structures have sufficient width such that they are relatively easy to fill with oxide. However, as device sizes decrease, isolation depth must be maintained while using trenches that are narrower. When these narrower trenches are filled with oxide, voids can form in the dielectric. These voids can result in poly bridging during subsequent process steps. More particularly, when the chemical mechanical polishing process opens up a void, poly-gate film can fill the void. The patterning process may not remove the poly-gate film within the void, creating undesired regions of polysilicon that can extend between adjoining lines and structures, causing current leakage.
Accordingly there is a need for a shallow trench isolation structure and a method for forming a shallow trench isolation structure that allows for the formation of narrow trenches that meet isolation depth requirements. Also, there is a need for a shallow trench isolation structure and a method for forming a shallow trench isolation structure that meet the above requirements, and that will not result in the formation of voids within the dielectric layer. The present invention meets the above needs.